To study the DNA excisional repair process in normal cells and in cells from patients with xeroderma pigmentosum (XP). Xeroderma pigmentosum (XP) is a rare autosomal disease in which the patients develop numerous malignancies on sun-exposed areas. Most patients with XP are unable to repair ultraviolet (UV)-induced damage to their DNA at a normal rate. We study the ability of XP patients' cells to repair UV-damaged DNA in vitro by measuring the rate of incorporation of radioactive thymidine into their DNA during the repair process by autoradiography and liquid scintillation spectroscopy. The XP patients are studied clinically with particular emphasis on evaluating the dermatologic, neurologic, ophthalmologic and endocrinologic manifestations of their disease. Attempts are made to correlate the patients' clinical findings with their DNA repair rate. In addition genetic complementation studies with cultured fibroblasts are conducted to determine whether the patients have similar or dissimilar genetic mutations which result in deficient DNA repair.